In response to environmental concerns, there has been an evolution from using freon and hydrochlorofluorocarbon foam blowing agents to hydrofluorocarbons, and eventually to carbon dioxide and/or hydrocarbons and alcohols. Unfortunately, as a result of this change, the thermal conductivity of foam material has increased due to the higher conductivity of these new blowing agents. This will result in insulation foams that no longer satisfy required product specifications unless additional steps are taken to increase the thermal resistance of these insulation foams.
It is known that the overall heat transfer in a typical foam block can be separated into three components: thermal conduction from gas (or blowing agent vapor), thermal conduction from polymer solids (including foam cell wall and strut), and thermal radiation across the foam block. Schutz and Glicksman, J. Cellular Plastics, March-April, 114-121 (1984). Of these three components, thermal radiation provides about one quarter of the overall heat transfer. Once the blowing agent and the polymer matrix are selected, it is difficult to affect the first two thermal conduction components, although they are important, occupying about 60% and 15% respectively to the overall heat transfer. Gas convection within the cells is negligible due to the small cell sizes present in typical insulating foam.
Heat radiation through polymeric foam materials is mainly in the format of infrared light. When a bundle of infrared light strikes the surface of an object, one part is reflected back into the environment, another part is absorbed by the object that is eventually transformed into heat or re-emitted back to the environment, and the rest is transmitted through the object. The infrared radiation emitted by an object is a function of its temperature. The wavelength of its peak intensity follows Wien's law, where the product of peak value wavelength and absolute temperature are held constant. As the temperature range of interest for plastic foams is around room temperature (i.e., 25° C.), this results in a peak intensity of infrared radiation of about 1000 cm−1.
An infrared attenuation agent (“IAA”) can be used to improve an insulating foam. An effective IAA favors increased reflection and absorption and decreased transmission of heat radiation as much as possible. Traditionally, flake-like inorganic materials have been used as the IAAs to reduce the portion of heat radiation. These include, for example, graphite, aluminum, stainless steel, cobalt, nickel, carbon black, and titanium dioxide. See Glicksman et al., J. Cellular Plastics, 28, 571-583 (1992). In commonly-assigned U.S. Pat. No. 7,605,188, the entire disclosure of which is incorporated herein by reference, surface-modified nano-graphite particulates that function as effective IAAs in polymer foams are described.
Unfortunately, one drawback of these inorganic materials is their incompatibility with relatively non-polar materials such as polystyrene. A relatively high weight percentage of these inorganic materials must also be used to achieve the required thermal resistance in the final insulating product. Because there is a limit to the amount of inorganic material that can be dispersed in a polymer foam, one cannot simply add higher amounts to provide the needed thermal resistance. Inorganic materials also tend to function as effective nucleation agents for polymeric foams, result in smaller cell size and higher foam density, which may be undesirable. There is therefore a need for infrared attenuation agents for use in insulating polymer foams that avoid these various processing difficulties while providing insulating foam having sufficient levels of thermal resistance.